How Much Energy Can You Really Get From a Coin Cell?

Motivated by what he considers outrageous claims made by a number of MCU vendors that their processors can run for several decades from a single CR2032 cell, Jack Ganssle compares coin cell batteries to see if the claims are legit.

About a year ago I wrote of my ongoing experiments to determine how coin cells behave. This was motivated by what I consider outrageous claims made by a number of MCU vendors that their processors can run for several decades from a single CR2032 cell.

Some vendors take their MCU's sleep currents and divide those into the battery's 225 mAh capacity to get these figures. Of course, no battery vendor I've found specifies a shelf life longer than a decade (at least one was unable to define "shelf life") so it's folly, or worse, to suggest to engineers that their systems can run for far longer than the components they're based on last.

Conservative design means recognizing that 10 years is the max life one can expect from a coin cell. In practice, even that will not be achievable.

There's also a war raging about which MCUs have the lowest sleep currents. Sleep current is, to a first approximation, irrelevant, as I showed last year.

But how do coin cells really behave in these low-power applications? I've been discharging CR2032s with complex loads applied for short periods of time and have acquired millions of data points.

My CR2032 experiment. A small ARM controller applies various loads to batteries being discharged and logs the results. (Source: Embedded.com/ Jack Ganssle)

The following results are for 42 batteries from Duracell, Energizer, and Panasonic. For each vendor I ran two groups of cells, each group purchased months apart from distributors located in distant states, in hopes that these represent different batches. (The devices are not marked with any sort of serial or batch numbers.)

It really depends on the application. This month I experienced two instances of battery failure in devices where sleep current largely defines the battery life. In both cases, a CR2025 is used to drive a micro that sleeps until a key is pressed.

The first instance, a DVD player remote control, died at about a year old. The control was found under some books, and had been there a while, likely pressing buttons the whole time. If that hadn't happened, I suspect the battery might have outlasted the player.

The second was the key fob transmitter on a 10 year old car. I've had the car since new, so I know the battery had never been replaced. The dead battery showed a few millivolts, and the new battery stopped working after a couple of days. I measured the sleep current, and it was way too high to support a 10-year battery life. Scrubbing the PCB with flux remover dropped the sleep current to a much more reasonable level. I was going to replace the battery in the second fob (which gets more use) but I think I'll wait until it (or the car) dies just to see what the lifetime is.

I think in both of these cases, the battery would normally have outlived the device, or at least the original owner, making it essentially maintenance-free to the original customer.

The statement about 10mA draw based on 200uA/MHz and maximum clock rate of 48MHz not being useful is pretty misleading. Quite a lot can be done with microcontrollers running at less than their maximum clock speeds, although wasteful programming and inefficient compliers can certainly result in the requirement to run at full speed. Efficient design practices generally payoff in less power, less rfi, less heat, and possibly smaller size/less weight.

The information about the internal resistance change is interesting. We saw a similar issue when looking for supercapacitors to power SSDs during power interruptions.

It depends on the application. I can think of one in particular in which the product stays plugged into the AC mains 24/7 and the coincell is needed only to retain certain data in RAM during periods of power outage or when the product is moved from one room to another, which is infrequently. These products typically last a decade or longer without depleting the coincell.